Animals exposed to the iron radiation particles ejected into space by supernovae show higher levels of plaque associated with Alzheimer's disease. The animals were exposed to about as much radiation as an astronaut would encounter while on a trip to Mars.

"It is extremely difficult from an engineering perspective to effectively shield against [iron particles]," said Kerry O'Banion, lead author of the study and a researcher at the University of Rochester Medical Center. "One would have to essentially wrap a spacecraft in a six-foot block of lead or concrete."

"Galactic cosmic radiation poses a significant threat to future astronauts," O'Banion said. "This is yet another factor that NASA, which is clearly concerned about the health risks to its astronauts, will need to take into account as it plans future missions."

A study published in March showed other effects from spending time in space. Of 27 astronauts tested, nearly all had some kind of eye or brain abnormality after their return from space, though there is no definitive connection to any long-term harm.

The earth's atmosphere and magnetic field shield the planet from most cosmic radiation. The International Space Station 200 miles above the planet's surface still benefits from that protection.

But astronauts who venture to other celestial bodies, like the Apollo astronauts who journeyed to the moon, were exposed. The hull of a spacecraft provides some protection, but high-energy particles can pass right through the shielding.

But those astronauts were only in space for a few weeks. Mars is much farther away, and with current technology, a one-way trip there is expected to take three years.

Before that happens, scientists might need to find a way to protect the passengers from the scattering remnants of exploding stars.